The apparatus for producing a stream of atoms or radicals by thermal dissociation of a gas as it traverses a heated tube through which the gas flows axially, is known and reference may be had to the article entitled "Some Properties of Hydrogenated Amorphous Silicon Produced by Direct Reaction of Silicon and Hydrogen Atoms", R. E. Viturro and K. Weiser in Philosophical Magazine B, 1986, Vol. 53, No. 2, pages 93-103.
For thermal dissociation of a gas at high temperatures, for example in the case of hydrogen or oxygen, the thermal dissociation temperature may have to be in the range of 1500 K to 2500 K. To be able to heat the gas passing through the tube to these high temperatures, conventional devices provide so-called electron bombardment heaters. In these, the electron source is a wire heated to glowing outside the tube and traversed by an electric current sufficient to cause the wire to emit electrons. A potential is provided between the electron source and the tube, e.g. of about 1000 volts, to accelerate the electrons toward the tube and cause them to impinge upon the tube so that a good part of the energy carried by the electrons is released on the tube and the gas flowing therethrough is thereby heated.
Electron bombardment has the advantage that a comparatively large amount of heat can be developed at the tube, i.e. that the heating power can be high, and further that the heat is concentrated at the tube rather than being dissipated. The electron bombardment technique, however, has the drawback that to generate the potential between the glow wire and the tube, the glow wire must be at close to ground potential and the tube must be brought to a high positive potential. In this case, the accelerated electrons can have in the region of the tube, sufficient energy for the impingement-type ionization of the gases which ultimately emerge from the tube as well as residual gas.
The resulting positive ions are accelerated away from the tube and thus there is an atomic beam which can cross the electron beam and dissipate energy.
In addition the beam emerging from the tube can be contaminated with ions from extrinsic sources because of the high energy of all of the atomic and subatomic particles involved in the process. Contamination of the beam of atoms or radicals may present a significant problem for the ultimate use of the beam.
It is possible to avoid contamination of the beam by ions by maintaining the tube at ground potential while providing the glow wire at a high potential. This arrangement has the drawback that it forms an "electron reflector" at the high potential which is so arranged that the electrons are accelerated only toward the tube and are not accelerated to further components also at ground potential.
The high voltage components must be sufficiently insulated and electron emission in extraneous directions must be suppressed, all of which significantly increases the fabrication cost of the apparatus and limits the use thereof in earlier systems. The need for high voltage insulation is particularly a problem since the components which must be insulated are also at high temperatures in practice. There is a significant danger that the insulator will be vaporized and the insulation effect lost.
Frequently it has also been found to be a drawback that the electrical source for electron bombardment heating comprises a high voltage unit which provides an electron emission current from about 60 mA to about 0.7 A, an electron-emission current regulator which must have positive feedback coupling between the tube temperature and the glow wire temperature to avoid unstable emission currents and other elements all of which contribute to a very high cost of the apparatus and introduce places at which failure can occur.